Re: [PATCH v3 0/4] KVM: Expose speculation control feature to guests

[Paolo Bonzini]

On 30/01/2018 04:00, David Woodhouse wrote:
> I believe Ashok sent you a change which made us do IBPB on *every*
> vmexit; I don't think we need that. It's currently done in vcpu_load()
> which means we'll definitely have done it between running one vCPU and
> the next, and when vCPUs are pinned we basically never need to do it.
> 
> We know that VMM (e.g. qemu) userspace could be vulnerable to attacks
> from guest ring 3, because there is no flush between the vmexit and the
> host kernel "returning" to the userspace thread. Doing a full IBPB on
> *every* vmexit would protect from that, but it's overkill. If that's
> the reason, let's come up with something better.

Certainly not every vmexit!  But doing it on every userspace vmexit and
every sched_out would not be *that* bad.

We try really hard to avoid userspace vmexits for everything remotely
critical to performance (the main exception that's left is the PMTIMER
I/O port, that Windows likes to access quite a lot), so they shouldn't
happen that often.

Paolo